1. Field of the Invention
The present invention relates to an optical head using an access mechanism of separation optical type.
2. Description of the Related Art
A conventional access mechanism of separation optical type comprises afixed.
A fixing optical unit a movable optical unit. A laser beam emitted from the fixed optical unit is reflected on a rotation mirror in the movable optical unit and changed in a direction which is a substantially perpendicular to an optical axis. The laser beam passes through a lens diaphragm and is condensed by an objective lens, and focused on a recording film on an optical disc. The laser beam reflected from the optical disc passes through the original passage, and is returned to a photo detector (not shown) provided in the fixed optical unit, and a signal, which is recorded in the recording film on the optical disc in advance, is read out. The movable optical unit is structured to be movable from an inner periphery of the optical disc to an outer periphery.
However, following problems are found in the conventional access mechanism.
The optical axis of the fixed optical unit and that of the movable optical unit are shifted. The objective lens is stopped by the lens diagram, so that the distribution of light density becomes asymmetric.
In a case where the distribution of light density becomes asymmetric, the output characteristic of a tracking error signal and that of a focus error signal are made offset. If the above asymmetry exceeds in an allowable value, there occurs trouble in accurately positioning the objective lens, and the signal is prohibited from being stably read.
Due to this, in the prior art, there were taken measures to considerably manage the accuracy of manufacturing the respective parts. However, it was difficult to assembly the apparatus with a sufficient accuracy.
In order to solve the above problem, there has been contrived a method in which alignment of the optical axis in a Y direction and that of the optical axis in a Z direction are controlled by adjusting an inclination angle of a light-transmission parallel plane, which rotates in a direction of a Z axis, and an inclination angle of a light-transmission parallel plane, which rotates in a direction of a Y axis. However, in this case, an adjusting range is narrow.
If the inclination angles of the parallel planes are enlarged, the adjusting range can be widened. However, in this case, luminous energy of transmission decreases and the phase changes, so that an optical characteristic may be deteriorated.